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Bohaskaia monodontoides
, a New Monodontid (Cetacea, Odontoceti, Delphinoidea)
from the Pliocene of the Western North Atlantic Ocean
Author(s): Jorge V?lez-Juarbe and Nicholas D. Pyenson
Source: Journal of Vertebrate Paleontology, 32(2):476-484. 2012.
Published By: The Society of Vertebrate Paleontology
URL: http://www.bioone.org/doi/full/10.1080/02724634.2012.641705
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Journal of Vertebrate Paleontology 32(2):476?484, March 2012
? 2012 by the Society of Vertebrate Paleontology
ARTICLE
BOHASKAIA MONODONTOIDES, A NEW MONODONTID (CETACEA, ODONTOCETI,
DELPHINOIDEA) FROM THE PLIOCENE OF THE WESTERN NORTH ATLANTIC OCEAN
JORGE VE?LEZ-JUARBE*,1,2 and NICHOLAS D. PYENSON2,3
1Laboratory of Evolutionary Biology, Department of Anatomy, Howard University, Washington, DC 20059, U.S.A.,
velezjuarbe@gmail.com;
2Department of Paleobiology, National Museum of Natural History, Smithsonian Institution, MRC 121 P.O. Box 37012,
Washington, DC 20013-7012, U.S.A., pyensonn@si.edu;
3Department of Paleontology, Burke Museum of Natural History and Culture, Seattle, Washington 98195, U.S.A.
ABSTRACT?Here we describe Bohaskaia monodontoides, a new taxon of beluga-like odontocete cetacean from the early
Pliocene Yorktown Formation of Virginia and North Carolina. Among odontocetes, Bohaskaia shares key characteristics of
the rostrum and face with belugas (Delphinapterus leucas), narwhals (Monodon monoceros), and Denebola brachycephala
from the late Miocene of Baja California, thus placing it as a member of the Monodontidae. It also displays autapomorphies
that merit its placement in a new genus and species. Both Denebola and Bohaskaia occurred in warmer latitudes than those
of extant monodontids, even accounting for extralimital records. Such data from the fossil record of Monodontidae indicates
that putative cold climate adaptations of living monodontids might have appeared under different environmental conditions
and that their Holarctic and sub-Arctic distribution is a relatively recent phenomenon.
INTRODUCTION
The only extant members of Monodontidae Gray, 1821,
include two iconic Holarctic cetaceans: narwhals (Monodon
monoceros) and belugas (Delphinapterus leucas). Both species
have been documented in Arctic and sub-Arctic waters (Stewart
and Stewart, 1989; Heide-J?rgensen et al., 2002), with occa-
sional extra-limital occurrences recorded in temperate latitudes
(e.g., Gladden et al., 1999). The fossil record of monodontids,
however, indicates that this group was represented by extinct
lineages that were distributed in lower, warmer latitude envi-
ronments (Barnes, 1984; Barnes and Deme?re?, 1991), challenging
panselectionist interpretations of monodontid specializations
as adaptations to Arctic climate (e.g., Noren and Williams,
2000; Williams et al., 2011). The oldest fossil monodontids
date to at least the late Miocene, with Denebola brachycephala
Barnes, 1984, from the late Miocene Almejas Formation of Baja
California, Mexico (Barnes, 1984). A Denebola-like form has
been reported from the late Miocene San Mateo Formation of
California, but it remains undescribed (Barnes and Deme?re?,
1991). This distribution suggests that monodontids belonging to
lineages other than extant ones had ranges that extended to tem-
perate latitudes at least by the late Miocene. With the exception
of a putative monodontid occurrence based on a single periotic
from the early Pliocene of Peru (Muizon and DeVries, 1985),
other early Pliocene and Pleistocene remains referable to mon-
odontids are known from the east coast of North America and
the North Sea Basin (Whitmore, 1970, 1994; Post, 2005; Lambert
and Gigase, 2007; Whitmore and Kaltenbach, 2008). Some of the
remains from the North Sea Basin consist of cranial material that
shares some similarities with Delphinapterus, although they are
too incompletely preserved for identi?cation below the family
level (Lambert and Gigase, 2007). Reported fossils from the east
coast of North America have been collected from the Yorktown
Formation in North Carolina and Virginia; the most notable of
*Corresponding author.
these ?nds includes material from the Lee Creek Mine, in North
Carolina, which consists of an associated collection of isolated pe-
riotics, a fragmentary mandible, and skull fragments (Whitmore
and Kaltenbach, 2008). Here we describe a new genus and new
species of monodontid based on a nearly complete skull from the
Yorktown Formation in Virginia, U.S.A., and refer some of the
aforementioned Lee Creek material to this new taxon.
Geological Setting?The type specimen USNM 25819 was col-
lected from the Rice?s Pit locality of the Morgarts Beach Member
of the Yorktown Formation, in Hampton, Virginia, U.S.A. Hazel
(1971) recorded that USNM 25819 was collected from Locality
11 (now known as Rice?s Pit), approximately 45?75 cm below
a Pleistocene shell bed (dominated by the marine bivalve Ensis
sp.) in a clayey sand. Rice?s Pit is an extensive open excavation
pit, about 13.2 m deep, which exposes marine sediments from the
Pleistocene Tabb Formation, overlying the thicker Yorktown
Formation, whose uppermost section is relatively fossiliferous
and has previously yielded a monachine seal, Pliophoca etrusca
(Koretsky and Ray, 2008), and the type specimen of Balaena
ricei Westgate and Whitmore, 2002. It is unclear if these latter
occurrences share the same precise stratigraphic provenance
as USNM 25819, although it is likely, given that the marine
mammal-bearing horizons at Rice?s Pit are restricted to the
uppermost, exposed horizons (Westgate and Whitmore, 2002).
Nearly all of the Yorktown Formation exposed at Rice?s Pit be-
longs to the Mogarts Beach Member (Dowsett and Wiggs, 1992),
based on the dominant presence of the planktic foraminiferans
Globigerina bulloides and Neogloboquadrina acostaensis. Also,
Hazel (1971) placed Rice?s Pit in the Orionia vaughani Zone,
based on that ostracode?s presence at the locality. Dowsett and
Wiggs (1992) reported an age estimate of 4.0?3.0 Ma, a range that
largely concurred with Cronin et al. (1984)?s placement of York-
town Formation localities from southeastern Virginia in standard
planktic foraminifer zones N19?21 and Blackwelder (1981)?s M5
molluscan zone for the Morgarts Beach Member of the Yorktown
Formation. Collectively, these biostratigraphic indicators point to
Piacenzian age for this new monodontid.
476
VE?LEZ-JUARBE AND PYENSON?-NEW PLIOCENE MONODONTID 477
Institutional Abbreviations?IRSNB, Institut royal des Sci-
ences naturelles de Belgique, Brussels, Belgium; M, Fossil mam-
mal collection of types and ?gured specimens of the IRSNB;
UCMP, Museum of Paleontology, University of California,
Berkeley, California, U.S.A.; UCR, Department of Geological
Sciences at the University of California, Riverside, California,
U.S.A.; USNM, Department of Paleobiology and Department
Vertebrate Zoology (Division of Mammals), National Museum
of Natural History, Smithsonian Institution, Washington, D.C.,
U.S.A.
Specimens Observed?Delphinapterus leucas (USNM A21051,
A21052, A22433, A23208, 238105, 275068, 572014); Monodon
monoceros (USNM A22983, A23455, A267957?267961).
SYSTEMATIC PALEONTOLOGY
CETACEA Brisson, 1762
ODONTOCETI Flower, 1867
DELPHINOIDEA Gray, 1821
MONODONTIDAE Gray, 1821
BOHASKAIA, gen. nov.
Type and Only Known Species?Bohaskaia monodontoides,
sp. nov.
Diagnosis?Same as that of the species.
Etymology?Honors David J. Bohaska, in recognition of his
extensive contributions to understanding the Cenozoic geology
and paleontology of the east coast of North America, and his life-
long commitment to enhancing the quality and care of natural
history collections in museums everywhere.
BOHASKAIA MONODONTOIDES, sp. nov.
(Figs. 2?5; Table 1)
Delphinapteridae, Whitmore, 1970:A145.
Delphinapterid, Hazel, 1971:12.
Delphinapterus Whitmore, 1994:225.
Delphinapterus sp., Whitmore and Kaltenbach, 2008:228?229 (in
part).
Holotype?USNM 25819, an incomplete skull, missing the left
supraorbital process, both zygomatic processes, the entire bas-
icranium, and both tympanoperiotics; collected by B. Burdette
and C. Carter in January, 1969.
Type Locality?Rice?s Pit, Hampton, Virginia, U.S.A. (Hazel,
1971). See Figure 1.
Formation?Morgarts Beach Member of the Yorktown For-
mation.
Age?Early Pliocene (3.8?2.9 Ma; Dowsett and Wiggs, 1992).
Referred Specimens?USNM 182963 (Whitmore and
Kaltenbach, 2008:?g. 71), anterior fragment of left maxilla
associated with an incomplete right zygomatic process of
the squamosal; collected at Lee Creek Mine, South Side of
Pamlico River, near Aurora Bath, by C. E. Ray, 7 November
1974.
Range?Pliocene of North Carolina and Virginia, U.S.A.
Diagnosis?Monodontidae based on (1) the medial exposure
of the maxillae surrounding the anterior and lateral margins of
the external bony nares, forming a ?keyhole window? re?ected
in rectilinear exposure of the maxillae anterior of the nares and
maxillary exposures rimming the anterolateral narial margins;
(2) lack of fossae for the pterygoid sinus in the orbital region
(also absent in Denebola); and (3) a lateral lamina of the palatine
that passes anterior of the lateral edge of the frontal groove.
Bohaskaia shares with Denebola: (1) a subcircular, blunt rostrum
in dorsal view, which differs from the squared, dorsoventrally
?attened rostral terminus of Delphinapterus and the broadly
triangular rostral outline of Monodon (ignoring the enlarged
maxillary alveolus for the tusk in males); (2) premaxillae that
TABLE 1. Measurements of holotype skull (USNM 25819) of
Bohaskaia monodontoides, gen. nov., sp. nov., in mm (after Perrin, 1975).
Length of rostrum?from tip to line across hindmost limits of
antorbital notches
259
Width of rostrum at base?along line across hindmost limits
of antorbital notches
216 e
Width of rostrum at 60 mm anterior to line across hindmost
limits of antorbital notches
151+
Width of rostrum at midlength 138
Width of premaxillae at midlength of rostrum 82
Width of rostrum at 34 length, measured from posterior end 112
Distance from tip of rostrum to external nares (to mesial end
of anterior transverse margin of right naris)
381
Greatest width of external nares 72
Greatest width of premaxillae 121
Projection of premaxillae beyond maxillae measured from tip
of rostrum to line across foremost tips of maxillaries visible
in dorsal view
15
Distance from foremost end of junction between nasals to
hindmost point of margin of supraoccipital crest
50
Length of upper left tooth row?from hindmost margin of
hindmost alveolus to tip of rostrum
154
Number of teeth?upper left 12
Number of teeth?upper right 11
Abbreviations: e, estimate; + , measurement on incomplete element.
dominate the anterior end of the rostrum, relative to the max-
illae, in dorsal view, differing from Delphinapterus, which has
rectilineal premaxillae and maxillae in equivalent widths, and
differing from Monodon, whose premaxillae taper with the max-
illae; (3) having the posterior end of the premaxillae reaching to
about mid-level of the external nares; and (4) palatines that are
separated in the midline by the maxillae.
Bohaskaia shares with both Denebola and Delphinapterus: (1)
pinching of the dorsal pro?le of premaxillae by anteromedial-
most dorsal infraorbital foramina, which undercut the lateral
margins of the premaxillae (this feature is absent in Monodon);
(2) shallow and planar premaxillary sac fossae, whereas those
of Monodon are convexly in?ated; and (3) teeth oriented an-
teroventrally. Bohaskaia exclusively shares with Delphinapterus:
(1) a window-like exposure of the vomer on the posterior of the
palate (unknown in Denebola); and (2) anteromedial sulci of the
premaxillae that extend anterior of the level of the antorbital
notches. It shares with Denebola and Monodon: (1) a narrow
V-shaped outline of the anterior margin of the exposure of the
maxilla medial to the premaxilla anterior to the external nares,
differing from the U-shaped outline of Delphinapterus; and (2)
apex of vertex composed of frontals.
Bohaskaia shares with Delphinapterus and Monodon: (1) an
absence of premaxillary teeth; (2) a raised rostral portion of
premaxilla, giving the rostrum a dorsally convex outline when
viewed laterally; and (3) long anterolaterally oriented frontal
groove. Bohaskaia exclusively shares with Monodon a vertex that
is constricted at the base, forming a knob dorsally, and rounded
dorsal outline of the nasals.
Lastly, Bohaskaia displays the following autapomorphies: (1)
contact between the maxillae and vomer in the posterior palate;
(2) open, or L-shaped, antorbital notches of the maxillae; (3)
cardiform exposure of the premaxillae in the palate; and (4) pro-
portionately wider external nares, relative to the breadth of the
skull; (5) frontal groove more anteriorly oriented relative to the
long axis of the skull (?33?); and (6) posterior accessory fora-
men and sulcus, positioned much further anteriorly, relative to
external bony nares, than in any other monodontid (although
related to the posterior extent of the premaxillae, which Bo-
haskaia shares with Denebola, this character is unclear in the
latter).
478 JOURNAL OF VERTEBRATE PALEONTOLOGY, VOL. 32, NO. 2, 2012
FIGURE 1. Locality map showing A, North America and B, the eastern Mid-Atlantic of the United States, showing the type locality (Rice?s Pit,
Virginia) and referred locality (Lee Creek Mine, North Carolina) for Bohaskaia. States of Delaware, Maryland, and West Virginia (DE, MD, and
WV, respectively) are also shown.
Etymology?Monodontidae, the crown clade of living nar-
whals and belugas; -oides, likeness (Latin). Named for its simi-
larities to both extant species of monodontids.
DESCRIPTION
Skull
The premaxillary sac fossae and vertex of the type specimen of
Bohaskaia are asymmetrical relative to sagittal plane of the skull
(as typical for many extant Delphinoidea) (Figs. 2?5). The ros-
trum is broad relative to the breadth of the skull, and the vertex
is skewed sinistrally (Fig. 2). The external nares are nearly circu-
lar in outline. Ventrally, the alveolar margins of the rostrum are
intact, but missing all dentition, and the palate is intact, except
that it lacks the pterygoids (Fig. 3).
Premaxilla?The premaxillae dominate the apex of the ros-
trum both in the type and referred specimens (USNM 25819 and
182963, respectively). Dorsally, the premaxillae are constricted at
the level just anterior to the antorbital notches, it then expands
laterally towards the apex of the rostrum. When viewed later-
ally (Fig. 5), the rostral portion of the premaxilla is notable for
its dorsally convex outline. A mesorostral groove separates the
premaxillae for their length anterior of the level of the orbits;
posterior of this level, the mesorostral groove is ?lled by the ossi-
?ed mesethmoid. The anteromedial sulcus of the premaxilla has
well-demarcated lateral margins. The premaxillary foramina are
located slightly posterior to the level of the antorbital notches;
the posterolateral sulcus of the premaxilla is shallow and contin-
ues for most of the length of the premaxilla posterior to the pre-
maxillary foramina. The premaxillary sac fossae are ?at with an
elongate, oval outline. As the premaxillae diverge laterally pos-
terior of the antorbital notches, they expose the underlying max-
illae along their medial margins anterior to the external nares.
The posterior ends of the premaxillae are located at the level of
the middle of the external nares; a small foramen is roofed by the
premaxilla at this level, and it is continuous with a deep sulcus
extending posteriorly along a longitudinal crest in the maxilla.
Bohaskaia does not possess premaxillary teeth. Although the
premaxillae dominate the apex of the rostrum in dorsal view,
they are excluded from the alveolar row by the maxillae in both
USNM 25819 (Fig. 4) and 182963. The palatal exposure of the
premaxillae is fairly long, reaching to about the level of the 11th
maxillary alveolus. Anteriorly, this exposure has a cardiform out-
line that pinches posteriorly (Fig. 5).
Maxilla?Dorsally towards the anterior part of the rostrum,
the maxillae are nearly completely covered by the premaxillae.
Mediolaterally, the dorsal transition between the maxilla to the
premaxilla is more abrupt than in extant monodontids, especially
in the raised portion of the premaxilla anterior to the antorbital
notches. Only the antorbital notch on the right side is preserved,
and it is wide, shallow and directed anterolaterally with an L-
shape outline; in extant monodontids they are deeper, U-shaped,
and oriented anteriorly. On the right side, USNM 25819 shows
a total of four anterior dorsal infraorbital foramina; the left side
is mostly missing, providing no commensurate information. The
largest of these foramina rank as the largest dorsal infraorbital
foramina in any monodontid, and it opens anterolaterally be-
tween the maxilla and premaxilla. The maxilla is expanded pos-
teriorly, covering most of the supraorbital process of the frontal
and temporal roof, except for a small portion posterolateral to
the vertex. The maxilla is exposed medially anterior to the ex-
ternal nares, which has been considered as a synapomorphy for
Monodontidae (Muizon, 1988), forming the anterior margins of
the external nares. The continuation of the small foramen that
exists posteriorly between the maxilla and the posterior-most ex-
tent of the premaxilla extends with a moderately deep sulcus in
the maxilla. The medial border of this sulcus slightly overhangs
the ?oor of the sulcus. This feature has not been named in the
literature (e.g., Mead and Fordyce 2009), and we elect to term it
the posterior accessory foramen and sulcus (Figs. 2, 5).
Ventrally (Fig. 3), the maxillae form most of the palate, which
is concave for most of its length, becoming ?at to slightly convex
posterior of the alveolar row. A total of 12 alveoli are present
on the right maxilla and 13 on the left; most are separated by
very shallow interalveolar ridges, except for the last alveolus on
VE?LEZ-JUARBE AND PYENSON?-NEW PLIOCENE MONODONTID 479
FIGURE 2. Dorsal views of the holotype skull of Bohaskaia monodontoides (USNM 25819), gen. nov., sp. nov. Abbreviations: adif, anterior dorsal
infraorbital foramen; ams, anteromedial sulcus; an, antorbital notch; fr, frontal; la, lacrimal; mrg, mesorostral groove; mx, maxilla; na, nasal; nuc,
nuchal crest; paf, posterior accessory foramen and sulcus; pdif, posterior dorsal infraorbital foramina; pf, premaxillary foramen; pls, posterolateral
sulcus; pmx, premaxilla; psf, premaxillary sac fossa; so, supraoccipital. Diagonal lines indicate broken surfaces.
the right, which has a deeper ridge. Judging by the orientation
of the alveoli, the upper teeth seemed to have been oriented
anteroventrally as in Delphinapterus and Denebola. The palatine
sulcus extends from the greater palatine foramen, at the level
of the anteromedial edge of the maxilla-palatine suture, to
about the level of the 9th maxillary alveolus; several other
small foramina are present along the length of the groove.
The maxillae contact the palatines posterolaterally, with two
long median extensions that separate the palatines along the
midline and contact the vomer posteriorly. These processes
are asymmetrical, with the right one being wider and slightly
longer.
Lacrimal?The lacrimals form most of the anterior edge of the
supraorbital process, and then extend posteromedially towards
the ventral infraorbital foramen (= maxillary foramen; Mead and
Fordyce, 2009).
Frontal and Parietal?The lateral margin of the supraorbital
process is straight-edged and arched dorsally. The frontals form a
pyramidal knob at the highest point of the vertex. This base of the
frontal portion of the vertex is constricted and pedestaled, with a
slight posterior tapering in dorsal view. On the ventral surface
of the supraorbital process the frontal groove is relatively long
and anterolaterally oriented, with an angle of about 33? from the
sagittal axis of the skull. There are no fossae for the pterygoid si-
nus in the orbital region. Only a small portion of the left parietal,
where it contacts the frontal, is present.
Nasal?The nasals are subequal in size, and rounded (button-
like) in outline when viewed dorsally.
Palatine, Vomer, and Pterygoid?In ventral view (Fig. 3), the
palatomaxillary suture is posteriorly concave, and the palatines
are separated in the midline by long interpalatine processes of the
maxillae. The lateral lamina of the palatines are not preserved,
480 JOURNAL OF VERTEBRATE PALEONTOLOGY, VOL. 32, NO. 2, 2012
FIGURE 3. Ventral views of the holotype skull of Bohaskaia monodontoides (USNM 25819), gen. nov., sp. nov. Abbreviations: an, antorbital notch;
fr, frontal; isc, internal sagittal crest; la, lacrimal; ma, maxillary alveoli; mx, maxilla; pa, parietal; pal, palatine; pmx, premaxilla; ps, palatal sulcus; s.ll,
sutural surface for lateral lamina of the palatine; so, supraoccipital; spt, sutural surface for pterygoid; ss, sagittal sinus; vo, vomer; voc, vomerine crest.
Diagonal lines indicate broken surfaces.
FIGURE 4. Anteroventral views of the rostrum of holotype skull of Bohaskaia monodontoides (USNM 25819), gen. nov., sp. nov. Abbreviations:
mx, maxilla; pmx, premaxilla.
VE?LEZ-JUARBE AND PYENSON?-NEW PLIOCENE MONODONTID 481
FIGURE 5. Lateral views of the holotype skull of Bohaskaia monodontoides (USNM 25819), gen. nov., sp. nov. Abbreviations: adif, anterior dorsal
infraorbital foramen; fr, frontal; isc, internal sagittal crest; la, lacrimal; ma, maxillary alveoli; mx, maxilla; na, nasal; paf, posterior accessory foramen
and sulcus; pal, palatine; pdif, posterior dorsal infraorbital foramina; pls, posterolateral sulcus; pmx, premaxilla; psf, premaxillary sac fossa; spt, sutural
surface for pterygoid; vo, vomer. Diagonal lines indicate broken surfaces.
although a sutural surface on the frontal, posterior to the lateral
edge of the frontal groove, seems to indicate that the lateral lam-
ina was expanded, as it is in other monodontids (Muizon, 1988).
The vomer is exposed in the posterior palate as a thin, elongate
window, as observed in Delphinapterus. The pterygoids are not
preserved.
Supraoccipital?Only a small portion of the supraoccipital is
preserved. It is concave and seems to have been nearly vertical.
The nuchal crest is unremarkable.
DISCUSSION
Comparisons?Besides the extant genera of monodontids, the
other available fossil comparisons with Bohaskaia are limited to
the type specimen of Denebola brachycephala (UCMP 321245;
formerly UCR 21245), described by Barnes (1984), and an inde-
terminate monodontid (IRSNB M. 1922) from the early Pliocene
of Belgium, reported by Lambert and Gigase (2007). Although
the type specimen of Bohaskaia is represented by a skull lacking
the basicranium, the preserved skull length from the rostrum
to the posterior surface of the vertex (?45 cm) makes it com-
mensurate in size with the largest skulls of adult Delphinapterus
and Monodon. This size is notably larger than the Denebola
type specimen, which represents an immature individual, based
on the lack of suture fusion between the basisphenoid and the
basioccipital. Viewed dorsally, the rostrum of Bohaskaia is
broad, compared to other monodontids, with laterally expanded
anterior terminations of the premaxillae that dominate the apex
of the rostrum, which differs from their anterior constriction
in Denebola and Monodon, and the rectilinear ones of Delphi-
napterus. The premaxillary sac fossae of Bohaskaia are ?at, like
Delphinapterus, and ovate, but not raised as in Monodon, nor
forming the bosses observed in phocoenids, albireonids, and
the Belgian monodontid (Barnes 1984, 2008; Fajardo-Mellor
et al., 2006; Lambert and Gigase 2007). The antorbital notches
of Bohaskaia are L-shaped, like Pontoporia (Pyenson and Hoch
2007), whereas most other monodontids possess U-shaped ones;
the Belgian monodontid approaches this condition (Lambert and
Gigase 2007:?g. 3). Viewed laterally, the rostrum of Bohaskaia
exhibits a strongly convex pro?le, whereas it is less convex in
extant monodontids and nearly ?at in Denebola (Barnes 1984).
Viewing the rostrum anteriorly, the alveolar margin of Bo-
haskaia is devoid of premaxillary teeth (Fig. 4), as in other extant
monodontids, with the exception of Denebola, which possesses
one alveolus per premaxilla (Fig. 6) (Barnes, 1984). Despite
the lateral expansion of the premaxillae on the dorsal surface
in Bohaskaia, the maxillae pinch medially to exclude the pre-
maxillae from the alveolar margin. Continuing in ventral view,
the curvilinear outline of the window exposing the premaxillae
is more laterally exaggerated in Bohaskaia than in Denebola
(Fig. 4, 6), but it is dissimilar to extant monodontids, where
the anterior portion of this outline is straight. In Bohaskaia,
the posterior termination of this window pinches in an acute
angle, whereas this pinching is more gradual and straight-edged
in other monodontids. The vomer is exposed in the posterior
palate in Bohaskaia, a condition similar to that seen in Delphi-
napterus; in Monodon, the pterygoids meet along the ventral
midline, thus covering the vomer in that region; this area is
482 JOURNAL OF VERTEBRATE PALEONTOLOGY, VOL. 32, NO. 2, 2012
FIGURE 6. Ventral views of the holotype skull of Denebola brachycephala (UCMP 321245). Abbreviations: an, antorbital notch; bo, basioccipital;
eam, external auditory meatus; fg, frontal groove; fpp, facet for posterior process; fr, frontal; hc, hypoglossal canal; la, lacrimal; ma, maxillary alveoli;
mf, mandibular fossa; mx, maxilla; oc, occipital condyle; pal, palatine; pf, palatal foramen; pma, premaxillary alveolus; pmx, premaxilla; pp; paroccipital
process; pt, pterygoid; rap, retroarticular process; zp, zygomatic process. Diagonal lines indicate broken surfaces.
not preserved in Denebola. The posterior palate of Bohaskaia
is similar to that of Denebola, with palatines separated in the
midline (Fig. 4, 6), differing from extant monodontids in which
these bones meet along a suture. In both extant monodontids
and Bohaskaia, the pterygoids are excluded from contacting the
maxillae by the palatines; in Denebola, the pterygoids contact
both the maxillae and the palatines (Fig. 6). The frontal groove
in Bohaskaia is long as in extant monodontids and Denebola.
However, in Bohaskaia it is more anteriorly oriented, forming
an angle of about 33? from the midline of the skull, which is less
than in Denebola (?50?), Delphinapterus (?47?), and Monodon
(?51?).
Like all other monodontids, Bohaskaia exhibits a key mon-
odontid character with the medial exposure of the maxillae an-
terior of the external bony nares. Posterolateral to the nares, the
posterior terminations the premaxillae of Bohaskaia are perfo-
rated by a foramen, which extends into a sulcus on the maxillae,
penetrating deeply enough to create a slight lip overhanging the
sulcus. We have identi?ed this accessory foramen and sulcus in
extant and fossil monodontids, as well as in phocoenids (Barnes,
1984; Lambert and Gigase, 2007; Ichishima and Kimura, 2009;
Figs. 2, 5), although we note that this structure was not identi?ed
in Mead and Fordyce?s (2009) compendium. The exact relation-
ship of this feature to other soft tissue structures in the cranium
remains unknown to us. Towards the vertex, Bohaskaia shares
button-like nasals with Monodon, which differs from the elon-
gate, chevron-shaped nasals observed in Denebola or the more
triangular ones of Delphinapterus. Also at the vertex, the frontals
of Bohaskaia form a pyramidal knob at the highest point, simi-
lar to Monodon and Denebola. The base of the frontal knob is
constricted and pedestaled as in Monodon, with a slight posterior
tapering in dorsal view.
Bohaskaia and the Belgian monodontid are of similar age
(Lambert and Gigase, 2007) and they do share some similarities
(i.e., outline of the exposure of the maxillae in front of the exter-
nal nares and the shape of the antorbital notch). However, other
comparable characteristics of the Belgian monodontid, such as its
smaller size, as well as features of the premaxillae, are different.
Bohaskaia and IRSNB M. 1922 could represent different species
of the same genus, but the fragmentary nature of the latter pre-
vents any further comparison.
Overall among monodontids, Bohaskaia exhibits features of
the premaxillae on the dorsal and ventral surfaces of the rostrum
that most closely parallel those of Delphinapterus, Denebola,
and the Belgian monodontid, while possessing a vertex that
shares more similarities with Monodon. This mosaic of features
suggests that Bohaskaia represents a distinct lineage of mon-
odontid, separate from existing genera. We view our detailed
differential diagnosis presented herein as a basis for conducting
a phylogenetic analysis of monodontid relationships, although
we have eschewed this step of our analysis primarily because
of the low number of ingroup taxa (four operational taxonomic
units to date, represented by each genus), and uncertain inter-
relationships among potential outgroup taxa (e.g., Delphinidae,
Phocoenidae, Albireonidae, and Odobenocetopsidae).
Biogeographic and Temporal Signi?cance?The presence of
monodontids in the early Pliocene of the eastern coast of North
America and the North Sea indicates a more widespread distri-
bution of this group in the Atlantic region. There is postcranial
evidence, of at least two, perhaps three, monodontids occurring
VE?LEZ-JUARBE AND PYENSON?-NEW PLIOCENE MONODONTID 483
in the Yorktown Formation at Lee Creek (Kaza?r and Bohaska,
2008); we propose that at least one of these morphotypes likely
belongs to Bohaskaia. Equally, one of the other Yorktown
taxa could belong to the Belgian monodontid. In either or
both cases, such a multispecies assemblage of monodontids
would represent occurrences of monodontids well south of the
published extralimital records for extant Delphinapterus (Reeves
and Katona, 1980).
Under the assumption that USNM 25819 represented Delphi-
napterus, Whitmore (1994) argued that its presence in the York-
town Formation represented winter immigration from higher
latitudes, rather than a lineage of more temperately adapted
species. Given that this occurrence represents a novel taxon
(Bohaskaia), either included or separate from a multispecies
assemblage of Pliocene age monodontids, we suggest an op-
posing view, arguing that these data represent faithful records
of in situ occurrence data, at least at the regional scale. First,
extant monodontids generally exhibit high latitudinal ?delity
even when breeding (Stewart and Stewart, 1989), and their death
assemblages likely have similarly high ?delity (Pyenson, 2011), a
supposition supported by abundant Pleistocene Delphinapterus
fossils from the former Champlain Sea covering southern Que-
bec, northern Vermont, upper New York State, and western
Ontario (Harrington, 1977).
Unlike the Holarctic and sub-Arctic distribution of living
monodontids, the geographic occurrences of their fossil relatives
are temperate, and possibly tropical latitudes. Estimates of
sea surface temperatures, based on planktic foraminifera, for
the Rice?s Pit locality range from 18.4?C to 27.3?C (Dowsett
and Wiggs, 1992) during the Pliocene. Late Miocene sea sur-
face temperatures in the eastern Paci?c Ocean, near the type
locality of Denebola on Isla Cedros, Baja California, Mexico,
were approximately the same as today, if not slightly warmer
during the summer (Moore and Lombari, 1981). Such warm
water geographic distributions for extinct monodontid lineages
suggest that putative cold-climate adaptations observed in extant
monodontids may have evolved recently in individual lineages
leading to Delphinapterus and Monodon, rather than re?ecting
ancestral cold water adaptive features Monodontidae.
ACKNOWLEDGMENTS
We dedicate this paper to Frank C. Whitmore, Jr., in recog-
nition of his ongoing in?uence and contributions to the study of
fossil marine mammals, through generations of students visiting
collections at USNM. We also thank the many volunteers and
collectors who have collected fossil material from Rice?s Pit lo-
cality in Virginia over many years. Careful comments and sug-
gestions from O. Lambert, an anonymous reviewer, and Editor
J. H. Geisler improved this paper. C. W. Potter and J. G. Mead
(USNM) provided access to modern specimens. J.V.J. thanks the
WBHR-LSAMP Bridge to the Doctorate Program for ?nancial
support during his ?rst two years at Howard University and a Na-
tional Museum of Natural History predoctoral fellowship from
the Smithsonian Institution. We also thank L. G. Barnes and
H. Thomas (LACM) for access to specimens and preparation
of key anatomical structures on the type specimen of Denebola
brachycephala, and L. G. Barnes, T. A. Deme?re?, and C. S. Gut-
stein for useful discussions. N.D.P. thanks R. Cavoukian for shar-
ing his lyrical observations on the diving ecology of juvenile
Delphinapterus at the Vancouver Aquarium, and L. G. Barrett-
Lennard, C. Birdsdale, and the staff at the aquarium for the op-
portunity to make observations on live Delphinapterus. N.D.P.
was supported with funding from the Remington Kellogg Fund
and the Smithsonian Institution.
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Submitted July 13, 2011; revisions received October 31, 2011; accepted
November 13, 2011.
Handling editor: Jonathan Geisler.